The present invention relates to digital communications systems and, more particularly to an encoding and decoding system in a disc drive.
In the field of digital communication systems, digital information is conveyed from a transmitter to a receiver through a channel. "Channel" is a generalized term that can include many different mediums. In data storage devices, such as magnetic disc drives, the channel includes a storage medium, and the digital information is transmitted to the storage medium and stored for some period of time before being recovered and delivered to the receiver.
A typical magnetic disc drive includes one or more rigid discs mounted for rotation on a hub or spindle. Each disc has an associated data head formed of a hydrodynamic bearing and a transducer, for communicating with the surface of the disc. An electromechanical actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a desired track on the disc surface for track following operations. A drive controller controls the disc drive based on commands received from a host system to retrieve information from the discs and to store information on the discs.
Information is typically stored in concentric data tracks on the disc surface. The direction of current through the transducer is controlled to encode magnetic flux reversals on the surface of the disc within the selected data track. In one type of coding, known as non-return-to-zero-inverse (NRZI) coding, a digital "1" is represented by a magnetic flux reversal from one bit position to the next in the data track, and a digital zero is represented by a lack of a magnetic flux reversal from one bit position to the next.
In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the desired data track, senses the flux reversals stored in the data track, and generates a read signal based on those flux reversals. The read signal is typically conditioned and then decoded by the drive controller to recover the data represented by the flux reversals.
All channels, including disc drive storage channels, introduce noise into the signals they convey. To detect and sometimes to correct signal errors caused by this channel noise, a large number of coding techniques have been developed. These coding techniques convert data words formed of a number of data bits into code words formed of a number of code bits. Additional bits in the code words permit the detection and sometimes the correction of errors in the signals received from the channel.
The ratio of the number of data bits to the number of code bits is known as the code rate of the code. In general, the ability to detect and correct errors in a received signal increases as the code rate decreases because a lower code rate means a greater number of additional bits in the code word. However, each additional bit added by the encoder increases the time and energy needed to transmit the signal through the channel. Thus, to minimize the time and energy needed to send the code, the code rate should be minimized.
Depending on the detection scheme, the code imposes certain constraints on the code word pattern. For example, in a disc drive, the rotational speed of the spindle motor that rotates the magnetic media varies over time. This results in non-uniform time intervals between read signal voltage pulses. A phase locked loop (PLL) is used to lock the phase and frequency of the read timing clock to the phase and frequency of the read signal voltage pulses. To ensure that the PLL is updated regularly, a code can be used that limits the number of consecutive zeros to no greater than a maximum number "k". This kind of code is known as a run-length-limited (RLL) code with a "k"constraint. The smaller the value of "k", the better the performance of the PLL. However, the smaller the value of "k", the more difficult the code becomes to implement.
The code may also limit the number of consecutive ones in an encoded value to limit the effects of inter-symbol interference, which occurs when consecutive transitions in the transmitted signal interfere with each other. Such codes are known as maximum transition run (MTR) codes with an "L" constraint, where L is the maximum number of consecutive transitions allowed in the channel signal. For example, to avoid three or more consecutive transitions, codes with an MTR constraint L=2 can be designed. Although MTR codes reduce inter-symbol interference, they eliminate a large number of available code words making it difficult and sometimes impossible to implement MTR constraints with high code rates.
The present invention addresses these and other problems, and offers other advantages over the prior art.